There are some financial advantages to being self-employed - your son would pay less National Insurance in total, and there is more scope for claiming expenses (such as the cost of travelling to work).
This must be weighed against the possible financial disadvantages of not receiving paid holidays, sick pay or redundancy compensation. Your son would have none of the protection offered by Employment Law as regards disciplinary procedure, unfair dismissal etc. He would also not have the guarantee of income, so he would need to draw up an agreement with the company stating the terms on which he offeres his services to them.
Being self-employed requires keeping records of all income and expenditure, to produce a statement of income to go with the annual tax return. It also requires the discipline to keep aside a proportion of income to pay a tax bill.
Employers have to pay 12.8% National Insurance contributions on top of an employees salary, and provide all the employee rights referred to above, so you imagine why it suits the company to have someone free-lance. You can also imagine that HMRC are very keen to stop this particular area of tax and NIC avoidance - if your son is working for one company solely, and is an employee in everything but name, then HMRC might decide that he is in fact an employee. They would normall go after the company for unpaid tax and national insurance, but it can be an unpleasant experience.
Finally, your son would need to check the insurance position - would he be covered by the company or would he have to provide his own. If he accidently damaged a vehicle, or was injured at work, it could be disastrous if he is not insured.
If I sound a little negative about this, then that is because in my experience it is the employer that has evrything to gain from these arrangements.
I would advise that your son asks a few more questions from the comapny before he accepts this.